In the German patent DE 199 03 439 A1 a procedure and a device to operate an internal combustion engine are described, in whose exhaust gas section a SCR catalytic converter (Selective Catalytic Reduction) is disposed, which reduces the nitrogen oxides contained in the exhaust gas of the internal combustion engine to nitrogen using a reagent substance. The metering of the reagent substance is carried out preferably as a function of the operating parameters of the internal combustion engine, such as, for example, the engine rotational speed and the amount of fuel injected. Furthermore, the metering is carried out preferably as a function of the exhaust gas parameters, such as, for example, the exhaust gas temperature or the operating temperature of the SCR catalytic converter.
Provision is made, for example, for ammonia to be the reducing agent, which is derived from a urea water solution. The dosage of the reagent substance or of the source materials of the reagent substance must be carefully established. A dosage which is too small has the consequence that the nitrogen oxides in the SCR catalytic converter can no longer be completely reduced. Too large of a dosage leads to a slip of the reagent substance, which on the one hand can lead to an unnecessarily large consumption of the reagent substance, and on the other hand as a function of the composition of the reagent substance can lead to an unpleasant odor.
In the German patents DE 199 60 731 A1 and DE 199 62 912 A1, NOx, sensors are described in each case, in which provision is made for the acquisition of the NOx concentration present in an exhaust gas flow. The NOx sensors contain multiple chambers, which are connected to each other via diffusion barriers. The known multiple chamber NOx sensors have as a result of the measuring principle a cross sensitivity with regard to ammonia (NH3). For example as a reagent substance, the ammonia contained in the exhaust gas leads to a falsification of the sensor signal by way of the reactions 4 NHS+5O2→4NO+6H2O. If an increase of the reagent substance dosage thus occurs during the previously known procedural approaches, the sensor signal will increase when an excess dosage or a correct dosage of the reagent substance exists due to the reagent substance slip which arises; and when an underdosage of the reagent substance exists due to the increasing NOx conversion, the sensor signal will drop out. If on the other hand the reagent substance dosage is lowered, the sensor signal will drop out if an excess dosage of the reagent substance exists due to the decreased reagent substance slip; and the sensor signal will increase when a correct dosage or an underdosage of the reagent substance exists due to the NOx conversion, which is no longer complete.
In the German patent DE 10 2004 046 640 A1, a procedure to operate an internal combustion engine and a device to implement the procedure are described, in which a NOx sensor with a cross sensitivity with regard to a reagent substance is disposed downstream after the SCR catalytic converter. At least one SCR catalytic converter, which is impinged with a reagent substance, is disposed in the exhaust gas section. The reagent substance contributes to the NOx conversion in the SCR catalytic converter. Provision is made for the calculation of at least one measurement for the NOx concentration arising downstream after the SCR catalytic converter. This calculation makes an increase in accuracy when establishing a dosage of the reagent substance possible.
A reagent substance slip can be ascertained from the difference between the calculated measurement for the NOx concentration and the measured quantity for the sum of the NOx concentration and the reagent substance concentration. The fact is taken into account that a reagent substance slip as well as an insufficient NOx reducing reaction causes a deviation in the same direction between the calculated measurement for the NOx concentration and the measured quantity for the sum of the NOx concentration and the reagent substance concentration. According to one embodiment the dosage of the reagent substance is initially reduced when a difference as mentioned above occurs. If a reagent substance slip were present, the reduction of the dosage of the reagent substance would lead to a reduction of the reagent substance slip. The reduction of the dosage of the reagent substance proved in this case to be the correct step. If too small a dosage of the reagent substance were originally present, the difference ascertained would continue to increase due to a small NOx, conversion, so that it could be suggested from this, that the reduction of the reagent substance was false and an increase of the dosage is to be conducted instead.
In the German patent DE 10 2004 031 624 A1, a procedure is described to operate a SCR catalytic converter used to purify the exhaust gas of an internal combustion engine, in which provision is made for an open-loop or closed-loop adjustment of the reagent substance fill level in the SCR catalytic converter to a specified storage set point. The targeted specification of the storage set point secures on the one hand that during transient states of the internal combustion engine, a sufficient amount of the reagent substance is available to remove the NOx emissions of the internal combustion engine before the SCR catalytic converter as completely as possible; and that on the other hand, a reagent substance slip is avoided.
The reagent substance fill level of the SCR catalytic converter is ascertained using a catalytic converter model, which takes into account the NOx mass flow entering into the SCR catalytic converter, the NOx mass flow departing the SCR catalytic converter, the temperature of the catalytic converter as well as if need be the reagent substance slip. The maximum possible reagent substance fill level of the SCR catalytic converter depends especially on the operating temperature of the SCR catalytic converter, which is the highest at low operating temperatures and drops off to lower values with an increasing operating temperature. The efficiency of the SCR catalytic converter depends on the catalytic activity; which is small at low operating temperatures, passes through a maximum with a rising operating temperature and drops off again when the operating temperature continues to rise.
The task underlying the invention is to indicate a procedure to operate an internal combustion engine, in whose exhaust gas section a SCR catalytic converter and a NOx sensor located downstream after the SCR catalytic converter are disposed, as well as a device to implement the procedure, which allows for the most optimal as possible result for the purification of the exhaust gas simultaneously with a minimal reagent substance slip.
The task is solved in each case by the characteristics indicated in the independent claims.